Next week we'll pour a footing for the garage addition. Most of it is 12x18, except 1 18' section that has a 12x18 base with a 8x24 stem wall on top, making the addition about 22" above the existing slab. I'll have to fill in the void in the middle of the new foundation with dirt/debris. Should the dirt/debris be compacted? What is the process and how long should the foundation walls cure before I fill, then pour the slab within the foundation?

This is the same project that started with a new slab section in an old garage. Drawings, permits, demolition of extensive termite damage. My wife's car is still in the driveway and there is no wall on the back of the garage. Give me about 2 more months . . .

not totally sure on the cure time, check with your concrete company, but i would say at least 3 days to a week.

the process for filling your new void is to get sand and rent a vibrating compactor. put in a couple of inches of sand and then run the compactor over it, repeat process until done. when getting ready to pour your garage slab i would highly suggest the use of rebar over the wire mesh, it is far superior in my opinion.

I'm sure someone on here can give you a definite cure time
good luck
Justin

I have enough #3 rebar to put in a 12x12 grid drilled and bedded into the footing. Will I still need wire mesh in addition to that? I didn't realize how extensive the filling would be. I've got a lot of dirt/small rock/dry clay chunk to put in the hole. Not exactly nice sand, but good enough?

When I do a foundation, we let it cure a minimum of 7 days before backfill. We also install the first floor deck to hold them together. With a garage wall and before installing the slab and back-fill we wait 14 days...ewww.
Then we back-fill in what is called lifts and keep heavy machinery away from the walls. The lifts are done 8 inches at a time and compacted for a garage floor. We also wet it slightly as we work.
Most folks go ahead and fill 3-4 days after, and just fill the hole then compact. Sometimes it works out...and sometimes there are really bad results. Concrete actually takes many, many years to cure...the Hover dam is still curing.

2 weeks sounds like a long time, but I'd sure hate to see a crack form from my impatience. I've got plenty of other related stuff to do that will still give me the satisfaction of seeing progress at the end of the day.

Yea, I'd say take care of other issues until a definite answer can be provided. I'm glad to see that you used the word "cure" versus "dry" a common mistake amongst the population. All I know is that the longer it takes to cure, the stronger it will be when done.

I think your rock, clay, and dirt will work. just be sure to do it a little at a time other wise you may end up with problems like inspectorD was saying. i would just use rebar, but I'm sure somebody else would say to just use the mesh.
Justin

Hello RobertE:
Concrete is allowed 28 days to gain its specified strength. If you use 3,000 pound concrete (common in footers and foundations) a test cylinder will withstand 3,000# of pressure after 28 days.
However, it does not have to have the full strength to hold the compacted earth and reinforced floor. I would go with InspectorD on the 7 day wait.
I like the idea of using the #4 re-bar at 1' mat instead of the wire mesh because the re-bar will also give temperature control over the concrete and it will be a much stronger floor. Just make sure the re-bar is spaced up with legs to make it stay in the center of the pour. So often the wire mesh is left on the bottom and doesn't reinforce anything.
Glenn